Naval architects and electronic warfare designers have expressed interest in large area conformal sonar arrays for applications where the transducers are electronically scanned to transmit and receive acoustic beams. These arrays would require thousands of transducers each separately mounted in a plane, but until now constructing an array in this fashion would result in high production and maintenance costs, large mounting areas and an unreliable design. As such, except in a very limited sense, larger transmitting/receiving arrays such as those found in phased array radar antenna technology generally or in conformal antennae technology particularly, have not been possible for sonar applications. However, conformal acoustic arrays hold extraordinary possibilities for undersea sonar applications such as 3-dimensional mapping, mine hunting and mine avoidance.
Large-scale array technology needs to solve several key problems before undersea operations can exploit its potential. The prior art includes arrays such as the bow array having transducers numbering in the hundreds and assembled using conventional technology. Bow arrays in cylindrical form are appendages to a ship and are incapable of a fully streamlined integration into a hull design. Also, to conserve space and to minimize hull penetrations, the drive, and receive circuitry of large arrays have had to be co-located in proximity to the transducers and outside the vessel's hull, requiring low voltages yet very high electric fields to exploit advanced transduction materials. Furthermore, array elements would have to be nearly identical in their input impedance characteristics in that the sheer numbers of elements contemplated would not permit individualized transformers/tuning circuits.
The curvature of the hull surface, to which a conforming array would be mounted, typically presents the designer with challenges due to dimensional instability and size. For example, expansion and contraction of such an array under environmental influences changes element-to-element separation. These types of problems tend to degrade the shape, gain, and sidelobes of electronically scanned beams. Accurate beamforming and shaping is therefore difficult to achieve because a ship's surface expands and contracts significantly due to density and temperature variations and tends to flex under the force of required maneuvering.
Sonar systems widely employ transmitting and receiving transducers utilizing a tonpilz configuration. These devices have a tail mass at a proximal end and a head mass at a distal end. Between these two ends piezoelectric ceramic element drivers extend longitudinally between and in physical contact with the head mass and the tail mass. A tie rod maintains the stack of drivers under a compressive stress. Excitation of the drivers at a frequency of resonance causes the head and tail masses to oscillate at a longitudinal frequency to provide a sonar signal.
Conventional tonpilz configuration technology has not been sufficiently adaptable to large-scale array applications, at least in part because conventional manufacturing processes make it difficult to control the input impedance that in some instances requires individualized transformers/tuning circuits. Further, the technology does not facilitate close electrode coupling due at least in part to the use of cemented joints between piezoelectric elements. Finally, requirements for electrode foils, cementing, and soldering when applied to the thousands of transducers required for one array, make the conventional technology impractical for many applications such as conformal transducer array applications.